Synthesis and characterization of violet light emitting polymer based on fluorene and dimethylsilane

Article abstract of DOI:10.1080/07315171.2013.803405

New fluorene based light emitting polymer, poly[(4-(9,9-didecyl-9H-fluoren- 2-yl) phenyl)dimethyl(phenyl)silane] (PFDPS), was synthesized by palladium-catalzed Suzuki coupling reaction. The obtained copolymer was characterized by 1H-NMR, and IR-spectrosco

Full text of DOI:10.1080/07315171.2013.803405

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Synthesis and Characterization of
Violet Light Emitting Polymer based on
Fluorene and Dimethylsilane
Dong-Soo Bae a , Dong-Hyun Lee a , Don-Eon Eim a , Seung-Eun Hu a ,
Shihaw Jung a , Ran Kim b , Jaeyoung Hwang c & Yun-Hi Kim b
a Gyeongnam Science High School , Jinseong , Jinju , Korea
b Department of Chemistry and Engineering Research Institute ,
Gyeongsang National University , Chinju , Korea
c School of Materials Science & Engineering and Engineering
Research Institute , Gyeongsang National University , Chinju , Korea
Published online: 02 Sep 2013.
To cite this article: Dong-Soo Bae , Dong-Hyun Lee , Don-Eon Eim , Seung-Eun Hu , Shihaw Jung , Ran
Kim , Jaeyoung Hwang & Yun-Hi Kim (2013) Synthesis and Characterization of Violet Light Emitting
Polymer based on Fluorene and Dimethylsilane, Molecular Crystals and Liquid Crystals, 578:1, 26-32,
DOI: 10.1080/07315171.2013.803405
To link to this article: http://dx.doi.org/10.1080/07315171.2013.803405
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Mol. Cryst. Liq. Cryst., Vol. 578: pp. 26–32, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/07315171.2013.803405
Synthesis and Characterization of Violet Light
Emitting Polymer based on Fluorene
and Dimethylsilane
DONG-SOO BAE,1 DONG-HYUN LEE,1 DON-EON EIM,1
SEUNG-EUN HU,1 SHIHAW JUNG,1 RAN KIM,2
JAEYOUNG HWANG,3,∗ AND YUN-HI KIM2,∗
1Gyeongnam Science High School, Jinseong, Jinju, Korea
2Department of Chemistry and Engineering Research Institute, Gyeongsang
National University, Chinju, Korea
3School of Materials Science & Engineering and Engineering Research Institute,
Gyeongsang National University, Chinju, Korea
New fluorene based light emitting polymer, poly[(4-(9,9-didecyl-9H-fluoren-2-yl)
phenyl)dimethyl(phenyl)silane] (PFDPS), was synthesized by palladium-catalzed
Suzuki coupling reaction. The obtained copolymer was characterized by 1H-NMR, and
IR-spectroscopy. The polymer showed good solubility in common organic solvents and
weight average molecular weight of 16,300 with polydispersity index of 1.4. The maxi-
mum photoluminescence of the solution and film of the polymer was observed at 392 nm
and 410 nm, respectively.
The double-layered device with the configuration, ITO/PEDOT/PFDPS/LiF/Al
structure has a turn-on voltage at about 5.5 V and maximum brightness of 9.40 cd/m2,
and emitted violet light at 414 nm.
Keywords Dialkylfluorene; dimethylsilane; PLEDs; violet light emitting polymer
Introduction
Polymeric light-emitting diodes (PLEDs) based on poly(p-phenylenevinylene (PPV) were
first realized in the 1990s [1]. PLEDs have been the subject of academic and industrial
research in recent years due to their potential applications in large area displays, flexible
display and general lighting [2–6]. For full color OLEDs display, it is very important to
develop wide-band gap materials for achieving high color pure and efficient blue emission.
Recently, the blue light emission is particular importance in the development of PLEDs.
Blue electroluminescence has been demonstrated in polycarbazoles, polyfluorenes, poly-
oxadiazoles, and polythiophenes [7–13]. Especially, poly(fluorene)s are attractive as active
components of organic light emitting diodes because of their thermal and chemical stability
∗Address correspondence to Jaeyoung Hwang, School of Materials Science & Engineering and
Engineering Research Institute, Gyeongsang National University, Chinju 660-701, Korea, E-mail:
jaeyoung@gnu.ac.kr and Yun-Hi Kim, Department of Chemistry and Engineering Research Institute,
Gyeongsang National University, Chinju 660-701 Korea, E-mail: ykim@gnu.ac.kr
26

New Violet Light Emitting Polymer
27
and their exceptionally high solution and solid state fluorescence quantum yields (0.6–0.8)
[14–18].
Moreover, the facile substitution at the 9-position of the fluorene monomer allows
control of polymer properties such as solubility, processability, and morphology. In this
regard, polyfluorenes are particularly promising for various optical applications. On the
other hand, the emission color of these polymers can be easily tailored by modifying the
molecular design of an organic material used in the polymer structures [16–21].
Thus, we designed new UV-violet light emitting polymer derived from fluorene and
dimethydiphenylsilane. The introduction of δ-silicon interrupts the π conjugation to achieve
UV-violet emission and inhibits intermolecular interaction because of its tetrahedral struc-
ture [22]. It is expected that the polymer can be deep blue material as well as host material
for blue OLED because of its high band gap. Furthermore, the polymer will have amorphous
nature due to its tetrahedral structure by introduction of dimethylsilane moiety.
The polymer was synthesized by well-known Suzuki coupling reaction. The thermal,
optical, and electrochemical properties of poly[(4-(9,9-didecyl-9H-fluoren-2-yl)phenyl)
dimethyl(phenyl)silane] (PFDPS) were investigated. The electroluminescence (EL) per-
formances of PFDPS were evaluated using device configurations: ITO/PEDOT:PSS/
PFDPS/LiF/Al.
Experimental
Materials
All reagents and solvents were purchased from Sigma-Aldrich Chemical Co. and TCI
Chemical. Tetrakis(triphenylphosphine)palladium(Pd(PPh3)4) was purchased from Strem.
Spectroscopic grade THF (Aldrich) was used for all absorption and emission experiments.
PEDOT was purchased from Bayer. All other compounds were used as received.
Bis(4-bromophenyl)dimethylsilane (1). 1,4-Dibromobenzene (10.0 g, 42.4 mmol) was dis-
solved in dry Et2O (300 mL) and cooled to −78◦C under nitrogen. n-BuLi (2.5 M in
hexanes, 17.0 mL, 42.4 mmol)was added into this solution dropwise and stirred for another
1 h at −78◦C. Dichlorodimethylsilane (2.74 g, 21.2 mmol) was then added dropwise at
this temperature. After the end of the addition, the solution was warmed slowly to room
temperature and then stirred overnight. The reaction mixture was quenched with H2O and
extracted with Et2O. The organic extracts were washed with brine and H2O, and then
dried over MgSO4. After the solvent was evaporated, the crude product was purified by
column chromatography (eluent = hexane/dichloromethane; 10:1 v/v). Yield: 29.8%. IR
(KBr, cm−1): 3100 (aromatic C H str), 2900 (aliphatic C H str), 1172(C-Br str): 1H-NMR
(300 MHz, CDCl3) δ 0.55 (s, 6H), 7.38 (m, 4H), 7.49 (m, 4H).
2,7-Dibromo-9,9-didecylfluorene (2). The compound was obtained by reported method
[18]. Yield: 39.7%. FT-IR (KBr, cm−1) 3100 (aromatic C H str), 2900 (aliphatic C H
str): 1H-NMR (300 MHz, CDCl3) δ 1.18 (m, 6H), 1.24 (m, 32H), 2.01 (m, 4H), 7.76 (m,
6H).
2,2ꢀ-(9,9-Didecyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (3).
The compound was obtained by reported method [23]. Yield: 64%. FT-IR (KBr, cm−1) :
1
3100 (aromatic C H str), 2900 (aliphatic C H str), 1345( B-O str), 1420(C-O str): H-
NMR (300 MHz, CDCl3) δ 1.18 (m,6H), 1.41 (s, 24H), 1.24 (m, 32H), 2.01 (m, 4H), 7.76
(m, 6H).

28
D.-S. Bae et al.
Poly[(4-(9,9-didecyl-9H-fluoren-2-yl)phenyl)dimethyl(phenyl)silane] (PFDPS). The pol
ymer was prepared from palladium catalyzed Suzuki coupling reaction. Tetrakis
(triphenylphosphine) palladium(0) (Pd(PPh3)4) (0.14 g, 10 mol%) was added to the mix-
ture of bis(4-bromophenyl)dimethylsilane (1) (0.53 g, 1.43 mmol) and 2,2ꢀ-(9,9-didecyl-
9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (3) (1.0 g, 1.43 mmol),
2M K2CO3 (3.0 mL) and anhydrous THF (4.0 mL) under nitrogen atmosphere. After the
reaction mixture was heated at 80◦C under nitrogen atmosphere for 24 hr, bromobenzene
was added, and then phenyl borate was added with small amounts of catalysts for end-
capping. Then the reaction mixture was poured into the solution of 2N HCl. Finally, the
crude polymer was dissolved in chloroform followed by the addition of methanol to afford
the desired polymer. Yield: 26.7%. FT-IR (KBr, cm−1) : 3100 (aromatic C H str), 2924
(aliphatic C H str): 1H-NMR (300 MHz, CDCl3) δ 7.66 (s, 14H), 0.67 (m, 6H), 2.05 (m,
4H), 0.78 (m, 32H), 0.85 (m, 6H).
Instruments for Characterization
1H-NMR spectra data were expressed in ppm relative to the internal standard and were
obtained on a Bruker Avance DRX 500 MHz NMR spectrometer. FT-IR spectra were ob-
tained with a Perkin-Elmer FT-IR spectrometer, and the UV-vis absorption spectra obtained
in chloroform on a Shimadzu UV 3100 spectrometer. Molecular weight and polydisper-
sity of the polymer were determined by gel permeation chromatography (GPC) analysis
with polystyrene standard calibration (Waters high pressure GPC assembly model M590
pump μ-styragel columns of 105, 104, 103, 500, and 100 Å, refractive index detectors,
solvent THF). Elemental analysis was performed by Leco Co. CHNS-932. TGA measure-
ments were performed on a Perkin-Elmer series 7 analysis system under N2 at a heating
rate 10◦C/min. The photoluminescence spectra were recorded on a Perkin-Elmer LS-50
fluoremeter utilizing a lock-in power meter (Newport 818-SL).
The cyclic voltammetry (CV) was performed using a CV-50W voltammetric analyzer
(Bioanalytical System Inc. (BAS, USA)). A glass carbon working electrode, a platinum
wire counter electrode and Ag/AgNO3 reference electrode were used in the three-electrode
system.
Fabrication of the LED. Poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene)
(PEDOT:PSS) for a conducting polymer hole-injection layer was coated on an indium-
tin oxide (ITO) coated glass substrate which had been washed with water, acetone, and
isopropyl alcohol sequentially. A thin polymer film was spin-coated (3000 rpm, 50 s) from
a filtered (0.2 m filter) 1.0 wt% polymer solution in chlorobenzene on a PEDOT:PSS layer.
A thin layer of LiF (0.5 nm) and subsequently a 100 nm layer of Al were evaporated
subsequently on the top of the emissive layer under a high vacuum (below 1 × 10−5 Torr).
All fabrication steps were performed in clean room conditions. Measurements were done
at room temperature in air.
Results and Discussion
The chemical structures and synthetic routes of monomer and polymer are shown in
Scheme 1. The polymer was synthesized by the Pd-catalyzed Suzuki coupling reaction of the
bis(4-bromophenyl)dimethylsilane and 2,2ꢀ-(9,9-didecyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolane). The molecular structures were verified by FT-IR and

New Violet Light Emitting Polymer
29
1H-NMR. The structure of polymer is consistent with the spectroscopy data from IR and
1
1H-NMR. In the H-NMR spectrum, PFDPS is confirmed by integration ratio of aro-
matic protons and aliphatic protons. In the IR spectra, the C H stretching of aromatic
and aliphatic protons appeared at 3100 and 2920 cm−1 and Si-C stretching appeared at
1450 cm−1. In the 1H NMR, aromatic protons appeared at 7.28–7.80 ppm, and Si-dimethyl
and other aliphatic protons appeared at 0.60–2.05 ppm, respectively. PFDPS was soluble in
common organic solvent such as chloroform, chlorobenzene, dichlorobenzene, and toluene
at room temperature. PFDPS has a weight average molecular weight (Mw) of 16,300 with
a polydispersity index (PDI) of 1.4 by a gel permeation chromatography (GPC) analysis
relative to polystyrene standards.
Scheme 1. Synthetic schemes of monomers and PFDPS.
The thermal properties of PFDPS were investigated through thermogravimetric analy-
sis (TGA) and differential scanning calorimetry (DSC), as shown in Fig. 1. The 5% weight
loss occurs at 436◦C for PFDPS, which indicates good thermal stability of the polymer
-3000
(b)
(a)
5% Weight loss : 436 o
C
100
80
60
40
20
-3050
-3100
-3150
-3200
-3250
90
100
110
100
200
300
400
500
600
700
Temperature (oC)
Temperature (oC)
Figure 1. TGA and DSC thermograms of PFDPS.

30
D.-S. Bae et al.
UV & PL (THF)
UV & PL (Film)
300
350
400
450
500
550
600
Wavelength (nm)
Figure 2. Absorption and photoluminescence spectra of PFDPS in solution and film.
(Fig. 1(a)). From the DSC curve, a distinct glass transition is observed at 101◦C, and no
other crystallization and melting peaks are found (Fig. 1(b)). The high thermal stability with
amorphous nature of the polymer prevents the deformation of the polymer morphology and
degradation of the polymer light emitting device by applied electric field of the LED.
UV-visible absorption and photoluminescence (PL) spectra of the polymer are pre-
sented in Fig. 2. Typical absorption spectra of dilute solutions of poly(9,9-dialkylfluorenes)
show with absorption maxima at about 375 nm. In THF solution, the absorption maxima
of PFDPS in blue-shifted to 335 nm compared to typical poly(9,9-dialkylfluorenes), due to
the presence of diphenylsilane moiety. The absorption edge of the film sample is at 403 nm,
which corresponds to the optical band gap (Eg) of 3.08 eV.
The emission maximum of the polymer in solution and solid film was 392 nm and
410 nm, respectively. The emission peak is only slightly broadened in the film state than that
in the solution state, resulting in the splitting of the excited states and more complicated
electron transitions. As a result, PFDPS showed a PL emission maximum at 410 nm,
yielding violet emission.
Cyclic voltammetry (CV) was employed to investigate the electrochemical behavior
as well as the HOMO and LUMO energy levels of polymer. CV was performed on a
film of polymer deposited on a carbon electrode measured in AgNO3 (0.01 M)/Bu4NBF4
(0.1M)/acetonitrile with a platinium counter electrode and silver reference electrode, cal-
ibrated against ferrocene-ferrocenium. The highest occupied molecular orbital (HOMO)
level of the polymer was calculated from their respective onset oxidation potentials accord-
ing to the equation HOMO = −(Eox,onset + 4.4) eV with the ferrocene oxidation potential
as the standard [24]; the lowest unoccupied molecular orbital energy (LUMO) levels could
only be estimated from the corresponding HOMO energy levels and optical band gaps (Eg).
The HOMO and LUMO energy levels for polymer is −5.83 and −2.75 eV, respectively.
The OLED device based on PFDPS with the configuration of ITO/PEDOT:
PSS/PFDPS/LiF/Al was fabricated. Poly(3,4-ethylenedioxythiophene) (PEDOT) doped

New Violet Light Emitting Polymer
31
10
10
8
(a)
(b)
0.00010
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Current density
Luminance
NTSC
8
0.00008
6
6
0.00006
4
4
0.00004
(0.187, 0.121)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
2
2
0.00002
0.00000
0
0
400
500
600
700
0
2
4
6
8
10
12
14
16
Wavelength (nm)
Voltage (V)
Figure 3. The current-luminescence-voltage characteristics (a) and EL spectrum (b) of the device
of polymer. The inset shows the CIE chromaticity diagram with the coordinate of the spectra for the
same device.
with poly(styrenesulfonic acid) (PSS) was used as the injection/transporting layer.
Figure 3(a) showed the current-luminescence-voltage characteristics of the EL device,
respectively. In the EL devices, the forward current increases with increasing forward bias
voltage, and the curve has a shape that is typical diode. The device has a turn-on voltage at
about 5.5 V and maximum brightness of 9.40 cd/m2. The electroluminescence spectrum is
displayed in Fig. 3(b). The device emitted violet light at 414 nm and a narrow full width at
half-maximum (FWHM) of 51 nm. The emitting color of device showed the violet emission
(x, y) = (0.187, 0.121) in CIE (Commission Internationale de lꢀEclairage) chromaticity co-
ordinates (inset of Fig. 3(b)). Bright violet electroluminescence was observed and the EL
spectrum is similar to that of PL spectrum.
Conclusion
We have synthesized new soluble violet light-emitting polymer, which is composed of
dialkylfluorene and dimethylsilane backbone by Suzuki coupling reaction. The polymer
shows good thermal stability. The introduced dimethylsilane in the main backbone induced
the polymer to have amorphous nature as well as to achieve UV-violet emission and inhibit
intermolecular interaction due to δ-silicon interrupts the π conjugation. The OLED with
the configuration of ITO/PEDOT:PSS/PFDPS/LiF/Al generated the maximum brightness
of 9.4 cd/m2 with a color violet emission of ELλmax = 414 nm.
Acknowledgments
This work was supported by KOFAC (R&E 2011) and the National Research Foundation
of Korea by the Ministry of Education, Science and Technology (2010-0029084).
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